fix(prime): update sling prompts to use gt prime --hook

Polecats weren't getting full role context (including IDLE POLECAT
HERESY guidance) because start prompts said `gt hook` instead of
`gt prime --hook`. The SessionStart hook already runs `gt prime --hook`,
but manual nudge messages were inconsistent.

Changed:
- sling_helpers.go: nudge message now says `gt prime --hook`
- startup.go: assigned beacon now says `gt prime --hook`
- startup_test.go: updated test expectation

Fixes: hq-iqyu9

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/sling_helpers.go

@@ -319,7 +319,7 @@ func injectStartPrompt(pane, beadID, subject, args string) error {
 	} else if subject != "" {
 		prompt = fmt.Sprintf("Work slung: %s (%s). Start working on it now - no questions, just begin.", beadID, subject)
 	} else {
-		prompt = fmt.Sprintf("Work slung: %s. Start working on it now - run `gt hook` to see the hook, then begin.", beadID)
+		prompt = fmt.Sprintf("Work slung: %s. Start working on it now - run `gt prime --hook` to load context, then begin.", beadID)
 	}
 
 	// Use the reliable nudge pattern (same as gt nudge / tmux.NudgeSession)